PURPOSE: To improve the lengthwise elasticity with an infiltration of a rubber material by twisting three or more of strands in which raw strand groups of two twisted strands are twisted mutually with single raw strands different in the diameter per strand to form a steel cord.

CONSTITUTION: Two raw strands 4 are S-twisted to form strand groups 5 which are S twisted with single raw strands 6 with the diameter thereof 1.2W2 times larger than the raw strand 4 to form a strand 7. Three or four of the strands 7 thus obtained are Z-twisted to form a steel cord 8. The steel cord 8 is excellent in the rubber infiltration rate (rubber infiltration sectional area/rubber infiltrable sectional area ×100), the fatigue resistance and the elasticity modulus as compared with the conventional type.